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Rise in filter pressure - just temporary

I've been using the "trick" to add DE to a Hayward filter for a few years now. I usually add about 2 cups with no notable change in filter pressure. Normal freshly-backwashed pressure is 20psi.

The pool's been open for about one week now. Lots of bather activity yesterday which severely clouded the water. Truthfully, the water was not 100% clear after opening last week. It was properly sanitized with bleach with no noticeable loss of chlorine prior to this weekend - just was not crystal clear yet after opening. I'm a bit of a fanatic with sparking pools.

After swimming yesterday, the pressure was at 34psi so I backwashed and pressure went to 20psi. Added DE. 3 hours later, the pressure was was 24psi! It was time to go to bed, so I set the time to cycle the filter off 6 hours later since, if the pressure continued to rise, I didn't want the pump operating with high pressure. I know using DE can cause rapid rises in pressure if small particles are present but never had a rise this quickly,

When I got up this morning and turned on the pump, I fully expected the pressure to be high and I'd have to backwash. But, the pressure had fallen back to a normal 21psi ! Weird. 12 hours later, the pressure is still reading 21psi.

So, my TFP friends, what can cause high pressure in the filter, but then after the pump was off for about 4 hours, the pressure drops back to normal (without backwashing). Something up with the filter? The filter is 7 years old. Never had issues. Have never changed the sand.

Some other stats: CYA 55, Cl 6, Ph 7.2, TA: 90. At the time pool had in it 1QT of Metal Magic and 4oz of Leslie's UltraBrite. I use both products about once a month but used both after opening.

Re: Rise in filter pressure - just temporary

DE can be like that. Everything gets totally plugged up, but once the pump turns off everything moves around a little and the DE can clump up, leaving clean spots that allow much less restricted flow when you turn the pump back on.

Re: Rise in filter pressure - just temporary

Sometimes, the DE (which I don't use) will quickly form a nice layer over the top surface of the sand thereby increasing the resistance to the water trying to flow through and, hence, the psi rise.

When you turn the pump off and on. that sudden "bump" in the flow and psi dislodges that delicate layer of DE and your pressure returns to normal as the DE doesn't re-cover the surface as effectively as it did prior to the "bump" of turning off and on.

EDIT: Sometimes I think Jason and I must be long lost twins based on the similarity of our responses but it couldn't be because I am so much, much younger.